Explain this joke

[Lirange]

"A biologist, a physicist, and a mathematician were sitting in a street café watching the crowd. Across the street they saw a man and a woman entering a building. Ten minutes later they reappeared together with a third person. “They have multiplied,” said the biologist. “Oh no, an error in measurement,” the physicist sighed. “If exactly one person enters the building now, it will be empty again,” the mathematician concluded."

I don't get it.